A Computer Program Listing Appendix on compact disc was submitted in this application. One compact disc and a duplicate copy were submitted containing the file xe2x80x9cPatent.asmxe2x80x9d, which was created on Nov. 12, 2001, and is 68,827 bytes in size.
This invention relates to synchronization of timing signals and to the transmission of encoded information such as for example the transmission of voice messages between remote transceivers in code.
It is known to transmit signals in digital code from one location to another for secrecy. However, most attempts to transmit digitized radio signals in encoded form in real time from one transceiver to another with reliable inexpensive equipment and good fidelity have been unsuccessful. This has been difficult to achieve with a coded signal because of the difficulty in synchronizing the encoding and decoding circuits in the transmitting and receiving apparatuses in real time with reasonable precision.
Accordingly, it is an object of the invention to provide a novel technique for synchronizing timing circuits not having a connection between them such as for example not having an electrical connection between them.
It is a further object of the invention to provide an electromagnetic linkage in which a signal is encoded and transmitted at one location and received and decoded at the other location without the use of a solid connection therebetween.
It is a further object of the invention to provide a novel radio for transmitting and receiving encoded signals.
It is a still further object of the invention to provide a novel portable transceiver capable of transmitting and/or receiving signals in encoded form from another source.
In accordance with the above and further objects of the invention, remotely located timing circuits such as oscillators operating at the same frequency are synchronized together from the signals that are transmitted fron one unit to the other. The signals, upon being received by the second unit, are used to synchronize the timing circuit in the second unit to that in the first unit to permit decoding of the data.
In the preferred embodiment, a transmitter digitizes and encodes an audio signal and then transmits the resulting digital coded signal to the receiver. The receiver decodes the signal and converts it back to an audio signal. The received encoded digital signal is used to reset the timing clock in the receiver so that the timing is synchronized in the receiver and transmitter by the transmitted encoded information. The synchronized signals are used in the encoding and decoding of the signal.
In one embodiment, the encoder feeds digital information to a shift register. Encoding switches select certain stages of the shift register and the transmitter transmits the exclusive OR of the digitized message at the output of the selected stages. This signal is also an input to the shift register. Thus, the information transmitted to the receiver is a scrambled digital signal. In the receiver, a shift register has the same stages selected as in the transmitter and the coded information synchronizes a clock for the shift register from the encoded digital signal that it receives. The selected stages of the shift register unscramble the coded information for conversion to an audio signal suitable for the speaker.
In the preferred embodiment, the transmitter converts the analog audio signal to a digital binary signal with 16 bit words. It counts the binary ones in each 16 bit word, and if the number of ones is an odd number, it inverts the digital word before transmitting with the entire encoded message. It transmits words having an even number of binary ones without inverting them. At the receiver, the words having an odd number of ones (or of zeros) is reinverted and the words are decoded using the same process as the coding.
Of course, other characteristics that can be determined from the transmitted word may be used to alter the digital code in a predictable manner. For example, if the coded words include an odd number of bits such as 17 bits but a binary one is always added at a fixed position before transmitting and always subtracted upon receiving, the bit in the fixed position would be ignored and the process repeated or an even number of bits per word could be used but binary zeros could be counted and the word invested if an even number of binary zeros are in the word as described above.
From the above description, it can be understood that the synchronizing technique of this invention and the coded transmission system of this invention have several advantages such as: (1) they provide low noise transmission; (2) there is very little cross talk between several different transmitters even when the encoding selections are only slightly different; (3) the equipment is simple and inexpensive; and (4) the code may be easily changed.
The above-noted and other features of the invention will be better understood from the following detailed description when considered with reference to the accompanying drawings, in which:
FIG. 1 is a block diagram of a radio system in accordance with an embodiment of the invention;
FIG. 2 is a block diagram of a cipher circuit used in the radio system of FIG. 1;
FIG. 3 is a schematic diagram of an encoder used in the cipher circuit of FIG. 2;
FIG. 4 is a schematic circuit diagram of a decoder used in the cipher circuit of FIG. 2;
FIG. 5 is a schematic circuit diagram of a synchronizing circuit for use in the cipher circuit;
FIG. 6 is a schematic circuit diagram of another embodiment of cipher circuit using a microprocessor;
FIG. 7 is a simplified schematic diagram showing one embodiment of encoding and decoding keys;
FIG. 8 is a block diagram showing a manner in which the circuit of FIG. 6 implements the invention;
FIG. 9 is a more detailed block diagram of a portion of the embodiment of FIG. 8;
FIG. 10 is a more detailed block diagram of another portion of the embodiment of FIG. 8;
FIG. 11 is a more detailed block diagram of another portion of the embodiment of FIG. 8; and
FIG. 12 is a more detailed block diagram of still another portion of the embodiment of FIG. 8.